The present invention relates to electrical diagnostic circuitry and more particularly to fuse blown detection circuits for use with programmable controllers.
Electrical devices such as programmable controllers frequently contain fuses in series with the control transistors in their output circuits. It is frequently useful for diagnostic purposes if a signal can be generated to indicate when this fuse is blown. In the past this has been accomplished through the use of a LED indicator in series with a resistor connected across the terminals of the fuse (in parallel). However, this construction is not always satisfactory because the LED and resistor allow for a certain amount of current leakage around the fuse itself. In the past this level of leakage, which might typically be several milli-amps has not posed a problem. However, with the increased sensitivity of many input modules to output devices which are hooked to programmable controllers this level of leakage is sufficient in many cases to signal the input module to turn on if the control transistor is on or is somehow shorted.
It is therefore an object of the present invention to provide a fuse blown detection circuit which does not allow any significant amount of leakage through the output circuitry of the programmable controller module it is associated with.
It is another object of the present invention to provide a low leakage detection circuit which can be readily adapted for use in both sinking and sourcing configurations.
It is a further object of the present invention to provide a low leakage fuse blown detection circuit which is straight forward in design and easy to implement.